Note about commit messages and changelog

[EricZolf]

Hello everybody,

in [1] there is a guidance to have changelog-relevant commit messages
prefixed with three capital letters (FIX/CHG/DEV etc) and a column [2].

Just to be clear, if you don't do this, your change(s) won't appear in
the changelog, and there will be no exceptions as the changelog is
generated semi-automatically with e.g. (see [3] for current output):

        tools/get_changelog_since.sh v2.0.0

It is the responsibility of the commit author to make sure that
meaningful information is provided with the commit(s), and it is the
responsibility of the merger to validate that this has happened, and to
fix it and even potentially reject a PR if this didn't happen.

I just have the impression that it hasn't happened always in recent
times, and want to avoid people being disappointed when release time
arrives, and their change doesn't appear in the changelog.

KR, Eric

[1]
https://github.com/rdiff-backup/rdiff-backup/blob/master/docs/DEVELOP.md#branching-model-and-pull-requests

[2] commit message lines irrelevant to the changelog, like technical
details, can and must still be without prefix.

[3] $ tools/get_changelog_since.sh v2.0.0
(make sure the version is the next correct one)

New in v2.0.1.dev13+g036a501 (2020-05-02)
----------------------------

## Changes

* DEV: clarify version tag pattern and their influence on releases,
closes #326
* FIX: fail with meaningful error message on metadata mirror files with
duplicate timestamps, closes #322
* NEW: option --allow-duplicate-timestamps to only warn about duplicate
timestamps in metadata mirror files, use this option with care and only
to clean an impacted backup repository.
* NEW: errorsrecovertest test script to test recovering from old errors.
* CHG: improved installation and contributors documentation
* FIX: avoid bytes/str object issue under MacOS/X while checking forks
FS abilities, closes #320
* FIX: sequence of exception leading to abort when logging tuple of
bytes because of unreachable directory, closes #310
* FIX: Update installation steps to make them clearer to users
* NEW: Create a new rdiff-backup-delete script which can remove a file
and all its history from a backup repository (use with care).
* FIX: Added backticks to `<file>` in develop docs so missing word is
shown, closes #303

## Authors

* albert-github
* Eric L
* Otto Kekäläinen
* Patrik Dufresne
* Trevor Harmon